1. Field of the Invention
This invention relates to an adaptive sensor array apparatus and a method of obtaining interference rejection.
2. Discussion of Prior Art
Arrays of sensors connected to associated signal processing units are well known. The sensors generate signals in response to received radiation for subsequent processing in the units to provide output signals. Each sensor signal is scaled and phase shifted by an associated weighting vector w in a processing unit to provide a corresponding conditioned signal. Conditioned signals from the sensors are summed in the unit to provide a processed output signal therefrom in a process known as beamforming. By phase shifting and amplitude scaling each of the signals in a controlled manner prior to combining them, the processing unit exhibits in its output signal a polar gain response to received radiation comprising one or more directions of enhanced gain and one or more directions of attenuation; the directions of enhanced gain are referred to as lobes or beams of the response, and the directions of attenuation as nulls thereof. By appropriate choice of the weighting vectors w, the contributions in the output signal arising from radiation from unwanted interfering sources within a field of view in which the sensors are receptive to radiation are at least partially cancellable relative to contributions arising from radiation from wanted sources therein. For this to be possible, the wanted sources must lie in different directions to the interfering sources relative to the sensors, so that response nulls are steerable towards interfering sources and lobes towards wanted sources.
In other words, the sensors and their associated processing unit exhibit a steerable polar gain response to radiation determined by the weighting vectors w. The vectors are calculable to provide enhanced gain in directions of wanted sources and reduced gain in directions of interfering sources. Values for the vectors w are calculable automatically under computer control from the sensor signals themselves to provide at least partial rejection of contributions in the output signal from interference sources, even when directions of arrival of radiation at the array are not known a priori.; this is known as adaptive beamforming, and is described in a publication xe2x80x9cAdaptive Array Principlesxe2x80x9d by J E Hudson, published by IEE and Peter Peregrinus, London 1981.
Apparatus incorporating arrays of sensors capable of adaptive beamforming are often operated on moving platforms such as aircraft and ships. As a result, the arrays are not always stationary with respect to wanted targets and unwanted sources of jamming and interfering radiation within fields of view of the apparatus.
There are a number of algorithms presently in use for computing the weighting vectors w described above. These algorithms rely on adjusting the vectors w gradually to track more slowly changing components of the sensor signals and assume that more rapidly changing random signal components are removed by integration and are hence not tracked. However, as disclosed in a publication xe2x80x9cA Kalman-type algorithm for adaptive radar arrays and modelling of non-stationary weightsxe2x80x9d IEE Conference Publication, 180, 1979 by J E Hudson, the assumption may be invalid for apparatus incorporating adaptive sensor arrays operating on future agile platforms which will be capable of performing more rapid trajectory changes in comparison to current platforms.
A more general solution than the Kalman-type algorithm for coping with rapid variations in the sensor signals is described by S D Hayward in a publication xe2x80x9cAdaptive beamforming for rapidly moving arraysxe2x80x9d, Radar 96, Beijing, China October 1996. In the solution, instantaneous weighting vectors wk for scaling the sensor signals are calculated from Eq. 1:
wk=wo+kxcex94wxe2x80x83xe2x80x83Eq. 1
where
wk=weighting vectors for use in scaling sensor signals to obtain an adaptive directional polar gain response;
k=a sample time within a time interval T during which the vectors wk are updated;
wo=initial values of weighting vectors wk; and
xcex94w=incremental weighting vector change for rapidly tracking a scene.
In the solution, the weighting vectors wo and xcex94w are calculated from a vector z using Eq.2:                               [                                    w              0                                      Δ              ⁢                              xe2x80x83                            ⁢              w                                ]                ≡        z                            Eq        .                  xe2x80x83                ⁢        2            
The vector z is in turn computed by solving Eq. 3:
Rz=xcex1Cxe2x80x83xe2x80x83Eq. 3
where
C=a matrix of constraints defining mainbeam gain direction;
xcex1=a scalar constant chosen to satisfy the constraints C; and
R=a covariance matrix of augmented sensor signal data as provided by Eq. 4:                     R        =                              1            T                    ⁢                                    ∑                              k                =                1                            T                        ⁢                          xe2x80x83                        ⁢                                          [                                                                                                    x                        k                                                                                                                                                                          x                          ∼                                                k                                                                                            ]                            [                                                                    H                                                        H                                                                                                              x                      k                                                                                                                          x                        ∼                                            k                                                                                  ]                                                          Eq        .                  xe2x80x83                ⁢        4            
xe2x80x83where
Xk=sensor signal data arriving at the sample time k;
X{tilde over ( )}k=augmenting sensor signal data including f(k) Xk where f(k) is a complex data scaling function which varies with the sample time k; and
H=a Hermitian transpose.
The function f(k) is chosen to match anticipated dynamic characteristics of a platform onto which apparatus implementing the solution is mounted for operation; it is often referred to as a penalty function. Although the solution can provide improved tracking of more rapidly changing scenes, it suffers a problem of providing poor cancellation of interference when there are multiple interference sources within a field of view of the apparatus. Moreover, the solution is more computationally complex than conventional solutions for adaptive beam forming.
Alternative solutions for computing the vectors w are described by Riba et al. in a publication xe2x80x9cRobust Beamforming for Interference Rejection in Mobile Communicationsxe2x80x9d, IEEE Trans. Sig. Proc., Vol. 45, No. 1 January 1997 and in a publication by Gersham et al. in a publication xe2x80x9cAdaptive Beamforming Algorithms with Robustness Against Jammer Motionxe2x80x9d, IEEE Trans. Sig. Proc., Vol. 45 No. 7, July 1997. In these alternative solutions, rapidly time-varying weighting vectors are not computed; instead, nulls in polar gain response provided by vectorially multiplying and then summing the sensor signals together are broadened in a slowly varying adaptive manner to ensure that sources of interference always lie within directions of the nulls. These alternative solutions have a disadvantage that a polar gain response of an apparatus provided thereby becomes unacceptably distorted when sources of jamming are located in a direction of a mainbeam response provided by the apparatus, or where there are multiple jamming and interference sources located in directions away from the direction of the mainbeam response where a residual polar gain sidelobe response is provided by the apparatus.
It is an object of the invention to provide an alternative adaptive sensor array apparatus providing enhanced interference rejection characteristics.
The invention provides an adaptive sensor array apparatus for generating an output signal in response to received radiation, the apparatus incorporating:
(a) multielement receiving means for generating a plurality of element signals in response to received radiation;
(b) processing means for processing the element signals to provide corresponding augmented signals in which element signals with and without such processing are grouped;
(c) adaptive computing means for adaptively computing weighting vectors from the augmented signals, and for processing the augmented signals using the weighting vectors to provide the output signal, characterised in that the processing means incorporates beamforming means for preconditioning the element signals when generating the augmented signals to enhance interference rejection characteristics of the apparatus when generating the output signal.
The invention provides the advantage of enhancing interference rejection characteristics of the apparatus by improving its performance to track sources of interference which are non-stationary relative thereto, and to attenuate its polar gain response in directions of these sources.
The beamforming means may be arranged to provide a first polar gain response for preconditioning the element signals and the apparatus may be arranged to provide a second polar gain response at its output signal, and a direction of enhanced gain in the first polar response may be aligned to a direction of enhanced gain of the second polar response. This provides an advantage against mainbeam interference jamming where the apparatus is used in a non-stationary environment.
The beamforming means may be arranged to provide a first polar gain response for preconditioning the element signals and the apparatus may be arranged to provide a second polar gain response at its output signal, and a direction of enhanced gain in the first polar response may be arranged to be substantially orthogonal to a direction of enhanced gain of the second polar response. This provides enhanced interference rejection characteristics compared to when a direction of enhanced gain in the first polar response is aligned to a direction of enhanced gain of the second polar response.
The beamforming means may be arranged to provide a first polar gain response for preconditioning the element signals and the apparatus may be arranged to provide a second polar gain response at its output signal, and a direction of enhanced gain in the second polar response may be arranged to be substantially in a direction of a null of the first polar response. This provides enhanced interference rejection characteristics compared to when a direction of enhanced gain in the first polar response is aligned to a direction of enhanced gain of the second polar response.
The processing means may be arranged to provide one or more processed signals and the apparatus may incorporate modulating means for modulating the processed signals to provide one or more modulated signals for grouping with the element signals to provide the augmented signals. This provides an advantage that modulation of the processed signals assists the computing means to compute the weighting vectors so that the apparatus is responsive to radiation from wanted regions of the scene and less responsive to radiation from unwanted regions thereof.
The apparatus may provide one modulated signal for grouping with the element signals to provide the augmented signals. This provides an advantage of reducing computation required in the computing means for calculating the weighting vectors.
The modulating means may be arranged to modulate the one or more processed signals using a signal adapted to match dynamic response characteristics of a platform bearing the apparatus.
The apparatus may incorporate analogue-to-digital converting means for digitising the element signals to provide corresponding digital signals, and the beamforming means and the computing means may be adapted to process the digital signals for generating the output signal.
The apparatus may incorporate data storing means for recording a plurality of sets of element signals, and the computing means may be arranged to calculate a corresponding set of weighting vectors from said sets of signals for use in generating said output signal.
In another aspect of the invention, a method of performing adaptive beamforming in an adaptive sensor array apparatus (100) is provided, the apparatus (100) incorporating a plurality of receiving elements (22), the method comprising the steps of:
(a) generating element signals in response to radiation received at the elements (22);
(b) preconditioning the element signals by beamforming them and then processing them to provide corresponding augmented signals in which element signals with and without such processing are grouped; and
(c) adaptively computing weighting vectors from the augmented signals, and for processing the augmented signals using the weighting vectors to provide an output signal,
thereby providing enhanced rejection in the output signal of contributions arising from interfering radiation received at the elements (22).